Some drive control apparatuses control driving of a brushless motor, which has no sensor for detecting a position of a rotor. For example, patent document JP-A-2011-360831 discloses one exemplary drive control apparatus, which controls driving of a brushless motor used as a drive source for a fuel pump.
In a case of a sensorless brushless motor, which is not equipped with a sensor for detecting a position of a rotor, the position of the rotor relative to a stator is not known under a state that the rotor is at rest before motor drive control is started. It is therefore necessary to start controlling driving of the motor after forcibly positioning the rotor to a control start position, which is the position of the rotor relative to the stator for starting the drive control. According to the drive control apparatus of the patent document, the position of the rotor relative to the stator is detected by switching over power supply to windings of plural phases. The control start position is determined based on a detected position of the rotor. The rotor is positioned to a determined control start position. Thus a period of time required before starting the drive control for the motor is shortened.
According to the drive control apparatus of the patent document, the rotor rotates in one direction (normal direction) or the other direction (reverse direction) depending on the position of the rotor relative to the stator at the time of power supply. In a case that the brushless motor is used as the drive source for the fuel pump, a shaft of the brushless motor is generally press-fitted into a hole part of an impeller so that the impeller is rotated by the brushless motor. An end part of the shaft and the hole part of the impeller are formed generally in an alphabetically D shape in section. To absorb manufacturing error and assembling error of the impeller, a certain clearance is provided between the end part of the shaft and the hole part of the impeller. For this reason it is likely that an angled corner part of an outer wall of the end part of the shaft collides a planar surface part of an inner wall of the hole part of the impeller with a certain acceleration when the shaft starts to rotate.
When the brushless motor of the fuel pump is driven by the drive control apparatus of the patent document, the shaft rotates in the normal direction or in the reverse direction each time the power supply to the windings is switched over at the time of positioning, which is performed before starting the drive control. It is therefore likely that the outer wall of the end part of the shaft collides against the inner wall of the hole part of the impeller more frequently and collision energy generated at the time of collision increases in comparison to a case that a brushless motor, which needs no positioning of a rotor and rotates in only a normal direction, is driven. Thus the impeller tends to wear and breaks. In an idle-stop vehicle and a hybrid vehicle, which are introduced more and more in recent years, a fuel pump is turned on and off more frequently in a certain period. For this reason, the collision of the shaft increases wear or breakage of the impeller. Further, the wear or the breakage of the impeller increases as the number of switchover of power supply to the windings at the time of the positioning increases.